Solid acid catalysts, including acid-washed bleaching earths, can be used in combination with an activator (such as an epoxide or a carboxylic acid anhydride) to initiate the polymerization of tetrahydrofuran. See, for example, U.S. Pat. Nos. 3,433,829, 4,189,566, and 4,243,799. Bleaching earths known in the art include naturally occurring aluminum hydrosilicates and aluminum/magnesium hydrosilicates of the montmorillonite type. The clays are normally activated by acid washing. When a carboxylic acid anhydride is used as the activator, the resulting polytetramethylene ether polymer has ester end groups. The ester end groups can be converted to hydroxyl end groups by base-catalyzed transesterification with an alcohol (see U.S. Pat. No. 4,230,892) or by catalytic hydrogenation.
A key disadvantage of the bleaching earth catalysts is that the polymers produced have higher polydispersities (M.sub.w /M.sub.n) than desirable, typically 3-4 at molecular weights of about 400 to 3,000. It is well known in the art that the molecular weight distribution (MWD) of the THF polymer impacts the properties of the polyurethanes or polyesters made therefrom. In general, mechanical properties of finished products are superior when THF polymers having a relatively narrow molecular weight distribution are used (see U.S. Pat. No. 4,933,503, column 2).
There are two general approaches to obtaining THF polymers having a relatively narrow MWD. In one approach, a THF polymer having a broad MWD is prepared, and the product is post-treated either by distillation to separate low molecular weight oligomers, selective depolymerization (see, for example, U.S. Pat. No. 4,363,924), selective solvent extraction with water/alcohol/hydrocarbon systems (see U.S. Pat. No. 4,762,951), or a combination of these techniques (see U.S. Pat. No. 4,933,503). Post-polymerization techniques are typically expensive, labor-intensive, and time consuming. A second general strategy is aimed at eliminating the need for post-treatment by preparing THF polymers having a narrow MWD. In one method, a low concentration of an alkylene oxide is maintained throughout the THF polymerization (U.S. Pat. No. 4,728,722). In another method, the mole ratio of the reactants and the reaction temperature are carefully controlled (U.S. Pat. No. 4,510,333). Unfortunately, these approaches are complicated and not completely satisfactory.
Other than acid-washed bleaching earths and some sulfonic acid resins, solid acid catalysts useful for preparing THF polymers in the commercially important 250-3,000 molecular weight range are not generally known. Copending application Ser. No. 07/839,538, filed Feb. 19, 1992, discloses a new class of THF polymerization catalysts: amorphous silica-aluminas. The silica-aluminas give THF polymers with commercially useful molecular weight distributions. Unfortunately, however, these catalysts tend to lose activity under conditions of continuous operation. In addition, some silica-aluminas will not ordinarily catalyze THF polymerization.